Get rid of unused server load balancing functions

[JRustyHaner]

Describe the bug
There's load balancing meteor functions that are not used. Server load monitoring really should be done with additional deployments using an offsite Mongo instance instead of writing software here. Meteor has this built in.

[imrryr]

Fixed in commit bac010e.

Removed the following unused server load balancing methods:

• updatePerformanceData() - was storing lastActionTimestamp and timestampLocation on user objects that were never being read
• isSystemDown() - was never called from any client code
• isCurrentServerLoadTooHigh() - was never called from any client code

Also removed all 7 client-side calls to updatePerformanceData() across:

• client/views/experiment/card.js
• client/views/new_templates/signIn.js
• client/views/new_templates/testlogin.js
• server/experiment_times.js

As noted in the issue, server load monitoring should be handled through deployment infrastructure with offsite Mongo monitoring rather than custom application code.